testscript: create a hidden temp directory by default

cmd/testscript/testdata/work.txt

@@ -9,11 +9,11 @@
 unquote file.txt dir/file.txt
 
 testscript -v -work file.txt dir/file.txt
-stderr '^temporary work directory: \Q'$WORK'\E[/\\]tmp[/\\]'
-stderr '^temporary work directory for file.txt: \Q'$WORK'\E[/\\]tmp[/\\]'
-stderr '^temporary work directory for dir[/\\]file.txt: \Q'$WORK'\E[/\\]tmp[/\\]'
-expandone $WORK/tmp/testscript*/file.txt/script.txt
-expandone $WORK/tmp/testscript*/file.txt1/script.txt
+stderr '^temporary work directory: \Q'$WORK'\E[/\\]\.tmp[/\\]'
+stderr '^temporary work directory for file.txt: \Q'$WORK'\E[/\\]\.tmp[/\\]'
+stderr '^temporary work directory for dir[/\\]file.txt: \Q'$WORK'\E[/\\]\.tmp[/\\]'
+expandone $WORK/.tmp/testscript*/file.txt/script.txt
+expandone $WORK/.tmp/testscript*/file.txt1/script.txt
 
 -- file.txt --
 >exec true

testscript/testdata/setupfiles.txt

@@ -1,6 +1,6 @@
 # check that the Setup function saw the unarchived files,
 # including the temp directory that's always created.
-setup-filenames a b tmp
+setup-filenames .tmp a b
 
 -- a --
 -- b/c --

testscript/testscript.go

@@ -289,13 +289,23 @@ type backgroundCmd struct {
 // It returns the comment section of the txtar archive.
 func (ts *TestScript) setup() string {
 	ts.workdir = filepath.Join(ts.testTempDir, "script-"+ts.name)
-	ts.Check(os.MkdirAll(filepath.Join(ts.workdir, "tmp"), 0777))
+
+	// Establish a temporary directory in workdir, but use a prefix that ensures
+	// this directory will not be walked when resolving the ./... pattern from
+	// workdir. This is important because when resolving a ./... pattern, cmd/go
+	// (which is used by go/packages) creates temporary build files and
+	// directories. This can, and does, therefore interfere with the ./...
+	// pattern when used from workdir and can lead to race conditions within
+	// cmd/go as it walks directories to match the ./... pattern.
+	tmpDir := filepath.Join(ts.workdir, ".tmp")
+
+	ts.Check(os.MkdirAll(tmpDir, 0777))
 	env := &Env{
 		Vars: []string{
 			"WORK=" + ts.workdir, // must be first for ts.abbrev
 			"PATH=" + os.Getenv("PATH"),
 			homeEnvName() + "=/no-home",
-			tempEnvName() + "=" + filepath.Join(ts.workdir, "tmp"),
+			tempEnvName() + "=" + tmpDir,
 			"devnull=" + os.DevNull,
 			"/=" + string(os.PathSeparator),
 			":=" + string(os.PathListSeparator),